Compact Optical Fiber Amplifier

by Isamu Oshima *, Katsuhiko Nishiyama *, Katsuya Kurotori * and Hirokazu Hayakawa *

Recently, WDM systems have been increasingly installed. As a result, we have ABSTRACT been developing optical fiber amplifiers for WDM systems. Compact optical fiber amplifiers for single channel and small channel applications are needed for SDH systems and for WDM systems to compensate for insertion losses of optical passive components. In response, we have developed compact optical fiber amplifiers using a coaxial pumping laser. However higher output power and higher gain have recently been demanded. Therefore, compact optical fiber amplifiers using a butterfly pumping laser or a mini-DIL pumping laser have been developed. These amplifiers are reported in this paper.

1. INTRODUCTION 2. LINE-UP OF COMPACT OPTICAL FIBER AMPLIFIERS The spread of the Internet is leading to increased communication traffic and requires the evolution of Dense Generally, optical fiber amplifiers are categorized accord- Wavelength Division Multiplexing (DWDM) technology. ing to signal wavelength ranges: C-band (approximately The Erbium Doped Fiber Amplifier (EDFA) developed 1530~1560 nm) and L-band (approximately 1570~1605 using a gain flattening filter technique 1), 2) and an expan- nm), or applications: booster amplifier, inline amplifier and sion of the signal wavelength range to L-band (approxi- preamplifier. mately from 1570 to 1605 nm) 3), 4) have supported this In the case of compact optical fiber amplifiers, they progress in the DWDM communication region. should be categorized by type of pumping laser: butterfly Compact optical fiber amplifiers for single channel and type and mini-DIL type. Mini-DIL pumping lasers do not small channel applications are needed. Their output need temperature control because the operating forward power and gain are not so high and they are mainly used current is not so high and they do not need a cooling in a single channel SDH system as a booster amplifier device. This contributes to reducing power consumption. and a preamplifier, or in the DWDM system to compen- On the other hand, butterfly pumping lasers need temper- sate for insertion losses of optical passive components ature control because the operating forward current is such as Optical Multiplexer (OMUX), Optical De- high to achieve a high pumping power. This difference is Multiplexer (ODMUX), Optical Add / Drop Multiplexer important because it has an impact on the design of the (OADM), and Optical Switch (OSW). As a compact optical electrical circuit. (In this paper the butterfly pumping laser fiber amplifier, we have developed a name card-sized and the mini-DIL pumping laser are called cooled pump amplifier using a coaxial pumping laser. However higher and uncooled pump, respectively.) output power and higher gain have recently been required Two platforms need to be designed. One includes the because of increasing losses from optical passive compo- cooled pump and the other includes the uncooled pump. nents due to increments in the number of signal channels The dimensions are the same, 90(W)×70(D)×12(H) mm. and complexity of configuring DWDM systems. To achieve The height of the compact optical fiber amplifier with the higher output power and gain, we have developed plat- uncooled pump can be reduced to 10.5 mm. forms as compact optical amplifiers using a butterfly Furthermore in the case of compact optical fiber ampli- pumping laser and a mini DIL pumping laser. fiers with the cooled pump, if a 1480-nm pumping laser is They are reported below. used, for instance, for L-band amplification, a heat sink is needed to radiate heat emitted by the laser diode and the cooling device, because the operating forward current is higher than in the case of using 980-nm pumping laser. So, a platform with a heat sink was developed. Table 1 shows a summary of platforms for compact optical fiber amplifiers. Next, the actual characteristics of compact optical fiber amplifiers are introduced according to the categories * Optical Subsystems Dept., FITEL Products Div. shown in Table 1.

Furukawa Review, No. 23 2003 71 Compact Optical Fiber Amplifier

Table 1 Line-up of compact optical fiber amplifiers. 0 °C 20 10 Division 2 Division 1 0 (Signal wavelength Notes -10 (Pumping laser) and application) -20 -30 Mainly C-band Booster- Low power consumption Uncooled pump -40 and Pre-amplifier 980-nm LD pump -50 Power (dBm/0.1 nm) -60 Mainly C-band Booster- High output power 950 955 960 965 970 975 980 985 990 995 1000 Cooled pump and Pre-amplifier Mainly 980-nm LD pump Wavelength (nm) 25 °C Cooled pump Mainly L-band Booster- Higher output power 20 with heat sink and Pre-amplifier Mainly 1480-nm LD pump 10 0 -10 -20 EDF -30 WDM IN CouplerIsolator coupler Isolator Coupler OUT -40 -50 Power (dBm/0.1 nm) -60 950 955 960 965 970 975 980 985 990 995 1000 FBG Wavelength (nm) Uncooled 980nm Input pump LD Output 65 °C monitor PD monitor PD 20 10 Figure 1 Optical configuration of amplifier with uncooled 0 pump LD. -10 -20 -30 -40 -50 Power (dBm/0.1 nm) -60 950 955 960 965 970 975 980 985 990 995 1000 Wavelength (nm)

Figure 3 Temperature dependence of output wavelength of uncooled pump LD with FBG.

the change of ambient temperature. See Figure 3. The spectra of the output of uncooled pump are shown in this figure. They are measured under conditions with ambient temperature of 0, 25, and 65¡C. They show that the wave- Figure 2 Appearance of uncooled pump LD (L) and cooled length is locked by the FBG in spite of the change of pump LD (R). ambient temperature, although the oscillation wavelength of the uncooled pump moves. (The curves near the bot- tom line are the output spectra of the uncooled pump.) 3. COMPACT OPTICAL FIBER AMPLIFIER Figure 4 shows the optical characteristics of the compact optical fiber amplifier with an uncooled pump. It is WITH UNCOOLED PUMP designed for a higher output power. The upper figure is The optical configuration of the compact optical fiber the required pumping power vs. signal wavelength, and amplifier with an uncooled pump is shown in Figure 1. The the lower figure is the noise figure vs. signal wavelength package size is 90×70×12 mm and some optical compo- when the input power is -10 dBm and the output power is nents such as uncooled pump, Photo Diode (PD), optical adjusted to +11 dBm. Output power of +11 dBm was coupler, optical WDM coupler, optical isolator, and EDF obtained under the condition that the signal wavelength are included in this package. The wavelength of the was from 1530 to 1566 nm and the ambient temperature uncooled pump is around 980 nm. was from 0 to 65¡C. The noise figure was less than 5.5 dB The appearance of the uncooled pump is shown in and the required pumping power was under 80 mW. Figure 2. The appearance of the cooled pump is also These results are summarized in Table 2. shown in the same figure. Figure 5 shows the optical characteristics of the com- As mentioned above the uncooled pump does not pact optical fiber amplifier optimized as a preamplifier. It is include a cooling device. So, the oscillation wavelength designed so that the gain is over 22 dB when the input changes according to ambient temperature. This behavior power is -25 dBm. The noise figure was under 5.7 dB. causes a degradation in the conversion efficiency of the These results are summarized in Table 3. amplifier, because the absorption band of EDF around Figure 6 shows a compact optical fiber amplifier with an 980 nm is very narrow. To avoid this phenomenon a Fiber uncooled pump. Bragg Grating (FBG) is set in the output fiber of laser. This FBG keeps the oscillation wavelength stable, in spite of

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Pump power Noise Figure Pin=-10dBm, Pout=+11dBm Pin=-25dBm,Gain=22dB 90 7.0 25°C 80 6.5 65°C 0°C 70 6.0

60 5.5

50 25°C NF (dB) 5.0 0°C 40 Pump power (mW) 65°C 4.5 30 1520 1530 1540 1550 1560 1570 4.0 Signal wavelength (nm) 1525 1530 1535 1540 1545 1550 1555 1560 1565 Wavelength (nm) Noise Figure Pin=-10dBm, Pout=+11dBm 7.0 Figure 5 Characteristics of compact optical fiber amplifier

6.5 25°C with uncooled pump LD (II). 0°C 6.0 65°C 5.5

NF (dB) 5.0 Table 3 Summary of compact optical fiber amplifier with uncooled pump LD (as a preamplifier). 4.5

4.0 Items Characteristics 1520 1530 1540 1550 1560 1570 Signal wavelength (nm) Operating temperature 0 to 65°C Signal wavelength range 1529 to 1563 nm Figure 4 Characteristics of compact optical fiber amplifier with uncooled pump LD (I). Input power -40 to -20 dBm Gain > 22 dB

Noise figure @ Pin=-25 dBm < 5.7 dB

Table 2 Summary of compact optical fiber amplifier with uncooled pump LD (as a booster amplifier).

Items Characteristics

Operating temperature 0 to 65°C

Signal wavelength range 1530 to 1566 nm

Input power > -10 dBm

Output power > +11 dBm

Noise figure @ Pin=-10 dBm < 5.5 dB

4. COMPACT OPTICAL FIBER AMPLIFIER WITH COOLED PUMP Figure 6 Appearance of compact optical fiber amplifier with The optical configuration of the compact optical fiber uncooled pump LD. amplifier with a cooled pump is shown in Figure 7. It is the same as that with an uncooled pump with the exception of EDF the pumping laser. The wavelength of the pumping laser WDM IN CouplerIsolator coupler Isolator Coupler OUT is around 980 nm. The butterfly pumping laser is in the same package as the mini-DIL pumping laser. This amplifier includes an input power and an output power monitor. Cooled 980nm Input pump LD Output A higher output power is achieved compared to the monitor PD monitor PD uncooled pump optical fiber amplifier. Figure 8 shows the optical characteristics of the com- Figure 7 Optical configuration of amplifier with cooled pump LD. pact optical fiber amplifier optimized as a booster amplifier. It is designed for an output power of over 15 dBm when the input power is -10 dBm. The upper figure is the required pumping power vs. signal wavelength, and the was from 0 to 65¡C. The noise figure was less than 5.0 dB lower figure is the noise figure vs. signal wavelength when and the required pumping power was under 150 mW. It the input power is -10 dBm and the output power is adjust- was confirmed that the radiation of heat was good and the ed to +15 dBm. An output power of +15 dBm was cooling device functioned well, even when the ambient obtained under the condition that the signal wavelength temperature was 65¡C. was from 1530 to 1561 nm and the ambient temperature These results are summarized in Table 4.

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Pump power Pout=+15dBm const. 170 160 150 140 130 120 25°C 110 65°C 100 -5°C Pump power (mW) 90 1525 1530 1535 1540 1545 1550 1555 1560 1565 Wavelength (nm)

Noise Figure Pout=+15dBm const. 6.0 ° 5.5 25 C 65°C 5.0 -5°C Figure 9 Appearance of compact optical fiber amplifier with 4.5 heat sink.

NF (dB) 4.0 3.5 3.0 1525 1530 1535 1540 1545 1550 1555 1560 1565 EDF WDM Wavelength (nm) IN Coupler Isolator coupler Isolator

Figure 8 Characteristics of compact optical fiber amplifier with cooled pump LD. Cooled 1480nm Input pump LD monitor PD Table 4 Summary of compact optical fiber amplifier with cooled pump LD (as a booster amplifier). EDF Isolator Coupler OUT Items Characteristics

Operating temperature -5 to 65°C

Signal wavelength range 1530 to 1561 nm Output monitor PD Input power > -10 dBm Figure 10 Optical configuration of amplifier for L-band. Output power > +15 dBm

Noise figure @ Pin=-10 dBm < 5.0 dB

A forward pumping configuration (without middle isolator) is one of the normal configurations of optical fiber 5. COMPACT OPTICAL FIBER AMPLIFIER amplifiers, but it has the weak point that the backward WITH A HEAT SINK Amplified Spontaneous Emission (ASE) wastes pumping power near the input terminal of the EDF, and this behav- A 1480-nm pumping laser is used for a higher output ior causes a degradation of the output power of the ampli- power amplifier and a L-band amplifier, because it is fier and noise figure characteristics. Therefore, the middle known that the efficiency of pump to signal conversion isolator is used to correct this. This middle isolator pre- when pumped by a 1480-nm laser is higher than that by vents the ASE from going back to the input end of EDF 980-nm laser. However the operating forward current of and wasting pumping power. Although this middle isolator the 1480-nm laser is apt to be higher and this causes an is designed so that insertion loss is lowest for the C-band, increment of cooler current. So, a heat sink is needed to the pumping and signal light can pass the isolator under radiate heat. A compact optical fiber amplifier platform the condition that losses at these wavelengths are only with a heat sink was developed for higher output power 0.2 or 0.3 dB larger than that of the C-band. For this rea- amplifiers and L-band amplifiers. The heat sink was opti- son, WDM couplers in front of and behind the middle iso- mized by simulating actual conditions, operating forward lator, which make the pumping light bypass the middle current, and cooler current, which depend on the output isolator, are not used. power of optical fiber amplifier. In the case of the optical fiber amplifier for L-band, it is Figure 9 shows an example of a compact optical fiber necessary to make the EDF much longer than for the C- amplifier with a heat sink. The dimensions are 90 (W)×70 band to obtain enough gain because the gain coefficient (D)×25 (H) mm. at the L-band is very small. The long EDF must be put into The characteristics of an optical fiber amplifier using this a compact package. To solve this problem a reduced platform are introduced in this section. cladding EDF was developed. The parameters of this EDF Figure 10 shows the optical configuration of the com- are shown in Table 5. Because the diameter of the normal pact optical fiber amplifier for L-band. An isolator is locat- EDF is 250 µm, the volume occupied is reduced to less ed between two EDFs; here, the ratio of these two EDFs than half. The EDF length was about 60 m. By increasing in length is about 1:10. (This isolator is called a middle the density of Erbium the EDF length can be shortened, isolator in this paper.) even if a higher gain is needed.

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Table 5 Parameters and characteristics of reduced cladding Table 6 Summary of compact optical fiber amplifier for L- EDF. band.

Items Characteristics Items Characteristics Mode field diameter 5.21 µm Operating temperature 0 to 60°C Cladding diameter 80±2 µm Signal wavelength range 1570 to 1610 nm Coating diameter 167±5 µm Input power > -10 dBm Cutoff wavelength 867 nm Output power > +13 dBm

Erbium density 1446 wtppm Noise figure @ Pin=-10 dBm < 5.7 dB Al density 3.3 wt%

Pump power Pin=-10dBm, Pout=+13dBm 180 ° 170 25 C 6. CONCLUSION 0°C 160 60°C 150 The following three platforms for compact optical fiber 140 amplifiers have been developed. 130 (1) Compact optical fiber amplifier with uncooled pump 120 (2) Compact optical fiber amplifier with cooled pump Pump power (mW) 110 100 (3) Compact optical fiber amplifier with heat sink 1565 1570 1575 1580 1585 1590 1595 1600 1605 1610 1615 Examples of optical fiber amplifiers using these plat- Wavelength (nm) forms have been reported. Noise Figure Pin=-10dBm, Pout=+13dBm Various compact optical fiber amplifiers for single chan- 6.5 25°C nel and small channel applications will be designed for 6.0 ° 0 C use with these platforms. 5.5 60°C

5.0

NF (dB) 4.5

4.0 REFERENCES 3.5 1565 1570 1575 1580 1585 1590 1595 1600 1605 1610 1615 Wavelength (nm) 1) P.F.Wysocki et al., OFC’97, PD-2, 1997 2) Tachibana et al., Society Convention of IEICE, C-3-43, p.143, 2002 Figure 11 Characteristics of compact optical fiber amplifier for (in Japanese) L-band. 3) M. Yamada et al., Electron. Lett., Vol.33, No.8, p.710-711, 1997 4) M. Fukushima et al., OFC’97, PD-3, 1997 5) Nishiyama et al., Society Convention of IEICE, B-10-53, p.341, 2002 (in Japanese) The characteristics of the compact optical fiber amplifier 6) Aiso et al., General Convention of IEICE, C-3-147, p.312, 2001 (in for the L-band are shown in Figure 11. It is designed for Japanese) an output power of over 13 dBm when the input power is - 10 dBm. The upper figure is the required pumping power vs. signal wavelength, and the lower figure is the noise figure vs. signal wavelength when the input power is -10 dBm and the output power is adjusted to +13 dBm. Output power of +13 dBm was obtained under the condition that the signal wavelength was from 1570 to 1610 nm and the ambient temperature was from 0 to 60¡C. The noise figure was less than 5.7 dB and the required pumping power was under 180 mW. It was confirmed that radiation of heat was good and the cooling device operated adequately even when the ambient temperature was 60¡C and the operating forward current was over 700 mA. These results are summarized in Table 6.

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